1. Field of the invention
The present invention relates to a thermoelectric conversion material composed of a composite of a novel bismuth-based thermoelectric conversion material and a permanent magnet material, which can be used in thermoelectric conversion elements and Peltier elements, and also relates to a thermoelectric (electrothermal) conversion element that makes use of this material and to a method for manufacturing this material.
2. Description of the Related Art
Thermoelectric conversion elements are devices that hold much promise for application in today""s industrial fields because of their efficient use of thermal energy. An extremely broad range of potential applications have been studied, such as in systems that use waste heat and convert it into electrical energy, small, portable power generators used to obtain electricity outdoors in a simple manner, flame sensors for gas devices, and so forth.
Known structures for thermoelectric conversion elements include one in which thermoelectric conversion materials exhibiting p- and n-type conduction are directly joined into an element by powder metallurgy, and one in which thermoelectric conversion materials exhibiting p- and n-type conduction are joined at a p-n junction with a metal such as silver solder.
Known thermoelectric conversion materials for forming such elements include IrSb3, Bi2Te3, PbTe, and other such high-performance chalcogen compounds, as well as FeSi2, SiGe, and other such suicides, which have a low thermoelectric figure of merit, but are found in abundance in nature.
It is well known that a thermoelectric conversion element converts heat into electricity by providing a temperature gradient between p- and n-type thermoelectric conversion materials, but that, conversely, it can also function as an electrothermal conversion element (that is, a Peltier element) that converts electricity into heat when voltage is applied to the above materials.
A conventional thermoelectric conversion element generates thermoelectromotive force (heat energy) by utilizing a temperature gradient (potential differential) imparted to materials, but the thermoelectric figure of merit (ZT=S2/xcfx81K; where S is the Seebeck coefficient, xcfx81 is the electrical resistivity, and xcexa is the thermal conductivity) of a thermoelectric (electrothermal) conversion element is only about 1 at best, which falls short of being satisfactory.
The conversion efficiency of a thermoelectric conversion element is far lower than that of a solar cell (about 20%), for example, topping out at just a few percent, and this is the main reason for the delay in the practical application of thermoelectric (electrothermal) conversion elements.
Meanwhile, the generation of an electrical field when a magnetic field is applied to thermoelectric conversion materials with a temperature gradient is known as the Nernst effect (L. D. Landau, E. M. Lifslitz, and L. P. Pitaevskli, xe2x80x9cElectrodynamics of Continuous Media,xe2x80x9d 2nd Edition, Pergamon Press, p. 101 (1984)).
With conventional thermoelectric conversion elements, though, the proposed structure was one in which a magnetic field was applied in order to increase efficiency in the conversion of the heat of a material into electricity, but this conversion efficiency was extremely low because of the low Seebeck coefficient of the thermoelectric conversion material.
It is an object of the present invention to provide a thermoelectric conversion material, and a thermoelectric conversion element that makes use of this material, which allow the Seebeck effect and the Nernst effect to be utilized synergistically, have a high Seebeck coefficient, and afford greater thermoelectromotive force.
As a result of various investigations aimed at achieving the stated object, the inventors found that the Seebeck effect and the Nernst effect can be utilized synergistically, and the Seebeck coefficient can be greatly increased, by mixing a bismuth-based thermoelectric conversion material containing one or more of the required dopants in an amount of no more than 5 at % in the bismuth, and a permanent magnet material such as an alloy powder for a rare earth magnet, and then applying a magnetic field to the compounded thermoelectric conversion material to magnetize the material itself.
The inventors perfected the invention upon discovering that a thermoelectric conversion element with a markedly higher Seebeck coefficient can be obtained by imparting a temperature gradient ∇T in the direction (z axis direction) perpendicular to the magnetization direction (x axis direction) of the above-mentioned thermoelectric conversion material, attaching an electrode material on the high- and low-temperature sides in a plane in the direction (y axis direction) perpendicular to the above two directions, creating a p-n junction, and extracting thermoelectromotive force from the connection end.
Specifically, the present invention is a thermoelectric conversion material that has a coercive force HcJ of at least 79.6 kA/m, wherein this material is a composite of a thermoelectric conversion material and a permanent magnet material, and more particularly one in which the thermoelectric conversion material is a bismuth-based thermoelectric conversion material containing one or more dopants in an amount of no more than 5 at % in the bismuth.
The above-mentioned thermoelectric conversion material is constituted such that the bismuth-based thermoelectric conversion material is a material that exhibits n-type conduction and contains at least one type of group VI element, rare earth element, alkali metal element, or alkaline earth element in an amount of no more than 5 at % in the bismuth, or p-type conduction and contains at least one type of transition metal element, group III element, or group IV element in an amount of no more than 5 at % in the bismuth, or is constituted such that the permanent magnet material, and particularly the alloy powder for a rare earth magnet, is an Rxe2x80x94Fexe2x80x94B-based alloy powder, an Rxe2x80x94Fexe2x80x94N-based allow powder, or an Rxe2x80x94Co-based alloy powder and its average particle diameter is 5 to 500 xcexcm.
Further, the present invention is a thermoelectric conversion element comprising means for applying a magnetic field in the required direction (x axis direction) of a thermoelectric conversion material, and thereby magnetizing this material; means for imparting a temperature gradient ∇T in the direction (z axis direction) perpendicular to the magnetization direction; and means for attaching an electrode material on the high- and low-temperature sides in a plane in the direction (y axis direction) perpendicular to the above two directions, creating a p-n junction, and extracting thermoelectromotive force from the connection end.
Further, the present invention is a method for manufacturing a thermoelectric conversion material, comprising the steps of mixing an alloy powder for a rare earth magnet with a bismuth-based thermoelectric conversion material containing one or more dopants in an amount of no more than 5 at % in the bismuth; and compounding this mixed powder by hot compression molding. More particularly, it is a method for manufacturing a thermoelectric conversion material constituted such that the hot compression molding is carried out in a magnetic field, or constituted such that the hot compression molding is hot pressing or discharge plasma sintering.
The thermoelectric conversion element pertaining to the present invention synergistically utilizes the Seebeck effect and the Nernst effect, resulting in a higher Seebeck coefficient and a greater thermoelectromotive force.
When the present invention constitutes a thermoelectric conversion element, there is no need for a device for applying an external magnetic field as in the past, which makes the thermoelectric conversion element itself more compact and lighter in weight, and also makes the element maintenance-free.